Despite advances in treating hematological malignancies, most patients either do not achieve remission or relapse after an initial therapeutic response. Nucleoside analogues (NAs), including arabinosyl cytosine (ara-C), fludarabine, cladribine, pentostatin, and more recently gemcitabine, troxcitabine and arabinosyl guanine (ara-G), are among the most important therapeutic agents currently used to treat hematological malignancies. Their antitumor activity depends on conversion to active, phosphorylated metabolites by intracellular kinases. Deoxycytidine kinase (dCK) catalyzes the rate-limiting phosphorylation step for the activation of all of these prodrugs. This application seeks (1) to develop a therapeutic system for delivery of dCK to the intracellular compartment to overcome the rate limiting step in NA activation, and (2) to engineer enzymes with improved catalytic activity for this therapeutic system. We will use ara-C as the model chemotherapeutic dCK substrate for these studies and an anti-CD33 antibody as the tumor targeting ligand. CD33 antigen is expressed by myeloid leukemia blasts, but not hematopoietic stem cells or other tissues. We will test the application that this "conjugate therapy" will increase the intracellular form of ara-C (ara-C triphosphate) and enhance its chemotherapeutic effect both in vitro and in vivo. We will manipulate enzyme structure with the goal of increasing both the efficacy and the therapeutic index of combined treatment with nucleoside analogues. Several tumor targeting antibodies are already in clinical use to internalize an attached protein, drug, or radioisotope into tumor cells. The selective delivery of enzymes with increased kinase activity that is proposed here has the potential advantage of reduced toxicity compared with radioisotope and drug systems. The long-term goal of this project is to develop methods to translate this Selective Enhanced Enzyme Delivery System (SEEDS) to applications for multiple types of malignancies.